1. Field of the Invention
The present invention generally relates to semiconductor substrate processing systems. More specifically, the present invention relates to a method and apparatus for performing etch process end point detection in a semiconductor substrate processing system.
2. Description of the Related Art
During the manufacture of semiconductor devices, a deep trench plasma etch provides non-mechanical separation simultaneously for all chips on a substrate (also referred to herein as a wafer). Being a highly productive process, a deep trench plasma etch process has found wide use in semiconductor wafer processing systems.
The term “deep trench plasma etch” is broadly used to refer to processes used to manufacture devices on silicon and non-silicon substrates comprising a processing step that plasma etches through a bulk of material of the substrate.
A requirement in such processes is a prompt termination of etching immediately after the first through, or clear, opening has been developed in the substrate. Therefore, reliable and accurate end point detection is critical during deep trench plasma etch. However, during deep trench plasma etch, the conventional end point detectors do not operate reliably.
There are generally two classes of background art systems for end point detection used during plasma etching, both require at least one viewing port in the etch chamber. The first class of systems includes laser interferometric detectors. These detectors focus a laser on the material to be etched and monitor the phase of the light reflected from the material. As the material is etched (removed), the phase of the reflected light changes in proportion with the depth of the etch. In this manner, the detector monitors the etch depth and can cause the etching process to stop upon achieving a predetermined depth. To measure minute phase changes, the equipment must be accurately calibrated, and such equipment requires repeated recalibration. Also, as line widths become narrower, maintaining the laser focus upon a bottom of a trench is becoming difficult.
The second class of the systems includes optical emission spectrometry (OES) detectors. These detectors comprise a data acquisition system and a plasma optical emission receiver and detect a change in intensity of one or several wavelengths of the plasma optical emission related to an etched or underlying layer. Sensitivity of these detectors reduces with either complexity of spectrums or intensity of the plasma as the spectral lines of interest become obscured by background spectrum.
To identify the occurrence of a deep trench plasma etch extending through the wafer, the change in the spectrum that occurs when backside gas escapes into the chamber through the trench is detected. For example, during the etch process, a backside gas (e.g., helium) is provided to the interstitial spaces between the wafer and the pedestal to promote heat transfer. As such, the gas leaks into the chamber from the edges of the wafer throughout the process. Therefore, the plasma always contains some amount of the backside gas. Upon the trench etching through the substrate, a small amount of additional backside gas escapes through the trench into the chamber. This additional gas alters the emission spectrum of the plasma. The end point detector can monitor this spectral change and stop etching upon its detection. However, the spectral change is so small that it might be missed until the etching process forms a substantial opening. Because the spectral change is small, any plasma non-uniformity may mask the signal.
If the end point is missed during deep trench plasma etch, there is a risk of plasma damage to the substrate pedestal and a risk of contamination of the pedestal by sub-products of the etching process and contamination of an etch chamber by a material from the pedestal. In many plasma chambers, a portion of the pedestal supporting a substrate is an electrostatic chuck that can be damaged even by accidental exposure to plasma or by the contaminants arising from exposure to the plasma.
Therefore, a need exists in the art for reliable end point detection during deep trench plasma etch.